Lymphoid tissue and lymphoid organs

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Peripheral (secondary) lymphoid organ

The spleen

  • It is the largest lymphatic organ. It is located in the upper left quadrant of the abdomen.

  • The stroma

    • It is covered externally by peritoneum (serous coat) exceptant the hilum.

    • Then its encapsulated by dense, irregular C.T (capsule).

    • Thin trabeculae extend into the parenchyma from the capsule. Trabecula also branched within the parenchyma of the organ.

    • Capsule + branching trabeculae consist of elastic fiber and smooth muscle.

    • In some mammals myofibroblasts contract to discharge stored red blood cells in the circulation because these species have the ability to hold large volume of RBCs in the spleen.

    • The stroma also contain reticular fibers + reticular cells which form a framwork around the parenchyma of the organ.

The parenchyma

      • The substance of the spleen consists of splenic pulp.

      • This is divided into white and red pulp, based on the color seen in fresh state.

      • The white pulp is islands on a sea of red pulp.

The white pulp = (Note: central only in the peripheral venul center.)

Malpigian corpuscles

The area where PALS formed, there is no nodular appear.

They appear as basophilic area in H&E due to present of a numerous lymphocytes those posses heterochromatic nuclei.

It is composed of aggregation of lymphoid tissue around the central arterioles forming corpusles.

Splenic artery traverse the capsule and branched in the trabeculae and then enter the white pulp, here the artery is called central artery (euentris fistion).

  • In some artery, around the central artery there is aggregation of lymphocytes which constitute the periarterial lymphatic sheath (PALS). It has cylindrical configuration around the central artery.

  • The PALS is differentiated from the lymphatic nodule by the presence of the central artery. True lymphatic nodules do occur. They appear as localized expansion of PALS and tend to display the center artery to the side (eccentric in position) rather than a central position.

Not be confused with lymphatic nodule which has similar appearance and the center in both may be called germinal center.
PALS T lymphocytes may be consider similar to the thymic dependent zone (of lymph node)

The germinal center of the lymphatic nodule of the white pulb mainly contains B lymphocytes.

T lymphocytes present toward the periphery at the junction of WP with RP. (marginal zone).

The red pulp

The parenchyma of the red pulp is composed of diffuse lymphoid tissue organized in cords which is highly infiltrated with blood cells mainly RBC. They are (cords) surrounded by splenic venous sinuses.

Therefore the red pulp consists mainly of splenic sinuses separated by splenic cords (cords of Biliroth).

The splenic cords mainly consists of reticular cells network and reticular fibers. This network or mestwork contains large number of RBC, macrophages, lymphocytes, plasma cells and granulocytes. Macrophages destroyed the old RBC. The venous sinuses lined by phagocytic reticular cells.

Blood supply

Capsular trabecular central artery within the white pulp. Then it terminate in the red pulp as highly branched small vessels termed penicillin, each penicillin showed a localized thickening of its wall that is called ellipsoid.

Blood from the splenic cord drain into the splenic sinuses drained into red pulp veins. They are present in between the splenic cords and have discontinues wall – red pulp vein join the trabecular vein splenic vein.

The circulation in the splenic is open because capillary open into the veins – venous sinuses are lined by reticular cells resemble macrophages not endothelium.

  • Marginal zone, red pulp cord (splenic cord) trap antigen and old blood cells. i.e. red pulp = blood filtration from macromolecule, antigen, senile cells.

  • Germinal center is involved in humoral immunity.

Antigen trap by macrophages of marginal zone + splenic cord

stimulate B cells toMarginal zone, red pulp cord (splenic cord) lead to trap antigen and old blood cells. i.e. red pulp = blood filtration from macromolecule, antigen, senile cells.

  • Germinal centers are involved in humoral immunity.

Antigen trap by macrophages of marginal zone + splenic cord

stimulate B cell to proliferate produce antibodies lead to attack


  • Role in cell –mediators immunity not clear.

  • Reservoir for blood – born elements such as lipid, irons, plasma protein.

  • Destruction of RBC.

Accessory spleen at the hilum or tail of pancreas in 16% of cases.

The lymph node

They are small encapsulated organs located along the pathway of lymphatic vessels. This association reflect their function as lymph filters.

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